Now I'm also requiring about this sensor part pictured below
because a few owner claimed to replace it and turned out to be the issue
And that could be the same problem for you as well. Now I'm just
following another forum on this, to ensure that this is the culprit
that caused my auxiliary fan to not running. If this the case than,
it would be one of the easiest repair than ever because this sensor is
located upfront just above the radiator on the inside, and it this NOT expensive at all.

Otherwise, this aux fan seems not that important on BMW's!
I guess you want to know why I say that?

1-These no such warning light/indication to tell you that aux fan has failed
Ex. check brake light, check coolant level, check fog lamp, washer fluid low

2-Found a few BMW's that comes with it, and some of them don't have it which are the same year/model as well.

(when I tested the head connector that bring the power, YES there's power there even when the engine's not running)

Attached Images